Electrolytic cell of the diaphragm type



Aug. 28, 1956 M. B. ALPERT ET AL ELECTROLYTIC CELL OF THE DIAPHRAGM TYPE2 SheetsSheet 1 Filed Jan. 51, 1952 MARSHALL B.A| PERT JAMES A.HAMIL'TON INVENTORS.

Fig. 2.

AG ENT Aug. 28, 1956 M. a. ALPERT ETAL 2,760,930

ELECTROLYTIC cm. OF THE DIAPHRAGM TYPE.

Filed Jan. 31, 1952 2 Sheets-Sheet 2 3 ARGO/v W Hz E| ARG 29% ai ARM Tifiiilii j IN VEN TORJ'.

MARSHALL b. ALPERT.

JAMES A.HAMILTON.

United States Patent (3 ELECTROLYTIC CELL OF THE DIAPHRAGM TYPE MarshallB. Alpert, Tomkinsville, N. Y., and James A. Hamilton, Keyport, N. 5.,assignors to National Lead Company, New York, N. Y., a corporation ofNew Jersey Application January 31, 1952, Serial No. 269,288

1 Claim. (Cl. 204-246) The present invention relates, in general, to theproduction of refractory metals by electrolytic means, and, moreespecially, to an improved electrolytic cell for the production ofrefractory metals of high purity from metal halides.

Electrolytic cells used for producing refractory metals may beclassified, in general, as of the diaphragmless type and of thediaphragm type respectively, the latter type of electrolytic cell, andthe one with which the present invention is particularly concerned,being characterized by two cathodes separated by one or more diaphragmsfrom an anode.

The development of this type of cell has been based upon the discoverythat while certain types of halides of refractory metals and inparticular the tetrahalides of titanium are not appreciably soluble inmolten salt electrolytes, the tetrachloride of titanium may be convertedelectrolytically to reduced titanium values i. e. the trivalent anddivalent chlorides, sometimes hereinafter referred to as titaniumtrichloride and titanium dichloride respectively, which are soluble in amolten salt electrolyte. When in solution in the molten saltelectrolyte, thesereduced titanium chlorides may be readily converted,by electrolysis, to titanium metal. In this connection the cathode whichis used to reduce the titanium tetrachloride to titanium dichloride and/or titanium trichloride is termed the solubilization cathode while thecathode upon the surface of which the reduced chlorides are deposited asmetallic titanium is termed the deposition cathode.

To insure the successful operation of the diaphragm type of cell, one ormore diaphragms are arranged in the cell so that the chlorine which isreleased at the anode is prevented from contacting either of thecathodes, or the electrolyte in the immediate vicinity of the cathodes(sometimes hereinafter referred to as the catholytes); while the reducedchlorides, which are formed at the solubilization cathode, are preventedfrom contacting the anode and the chlorine atmosphere surrounding thelatter.

In accordance with the present invention it has been found that whereasin this type cell the ratio of the trivalent and the divalent titaniumvalues is fairly low, i. e. of the order of one to four and remainssubstantially constant during convection from the solubilization cathodeto the deposition cathode, it is desirable that a concentrationdifferential of the low valent titanium values be ice 2 commercialproduction of refractory metals of superior quality.

Another object of the invention is to'provide an improved electrolyticcell of the type hereinabove described having diaphragms of improveddesign and arrangement whereby a high concentration differential ofreduced maintained between the catholytes of the solubilization cathodeextends.

titanium values may be produced and maintained in the electrolyte.

A still further object of the invention is to provide a superiorelectrolytic, cell of the type hereinabove described wherein thediaphragms of the cell are so designed and arranged relative to eachother and to the cell bath that segregated regions of high and lowconcentrations of trivalent and divalent titanium values may bemaintained in the bath thereby insuring high efficiencies and acommercially salable titanium metal.

These and other objects and advantages will appear to those skilled inthe art from the following description considered in conjunction withthe accompanying drawings.

In the drawings in which certain modes of carrying out the presentinvention are shown forillustrative purposes:

Fig. 1 is a side elevation, partly in section, of the improvedelectrolytic cell of this invention showing the design and arrangementof the diaphragms of the cell;

Fig. 2 is a plan view of the electrolytic cell on section line 22 ofFig. 1;

Fig. 3 is a side elevation, partly in section, of a modification of thecell of Fig. l; and,

Fig. 4 is a plan view, in section, of the modified cell of Fig. 3.

Referring to Figures 1 and 2 of the drawings, the improved electrolyticcell of this invention is shown by way of example as comprising an outercasing 10 provided with a removable cover 11 having a jacketed stack 12.The casing 10 may be formed of a corrosion resistant metalsuch as, forexample, Inconel (which is an alloy consisting essentially of nickel,chromium and iron), or a metal having similar properties and is asubstantially cylindrical cup-shaped member provided at its upper openend with an annular outwardly projecting flange 13 for supporting thecover 11. Mounted within the cell casing 10 is a substantiallycylindrical cup-shaped lining 14 which may be formed of silica, or anequivalent material.

The cover 11 of the cell is'mounted on the flange 13 of the casing, thejoint being sealed by a suitable asket, and comprises, preferably, ahollow substantially disc shaped member provided with a plurality ofsubstantially U-shaped indentations 15 arranged radially in theperiphery thereof for accommodating fastening means for detachablysecuring the cover to the flange of the cell casing. As shown especiallywell in Fig. 1 the fastening means may comprise bolts 16 hingedlysecured at their lower ends to the underside of the cell casing flange13 and arranged to extend upwardly through the aforesaid indentations 15of the cover and to be engaged at their upper ends by wing nuts 17. Thecover is made hollow so that it may be cooled during the operation ofthe cell and to this end the cover is provided with inlet and outletpipes 18 and 19 by which a coolant may be circulated through the cover.

The solubilization cathode of the cell is indicated at 20 and comprisesa metaltube, which may be formed of nickel or an equivalent materialadapted to be supported at its upper end in the cover with its lowerendextending to a suitable depth into the electrolyte of the cell. Thesupporting means for the upper end of the solubilization cathode 20 ispreferably a metal sleeve 21 secured with a fluid tight fit invertically aligned apertures in the upper and lower plates of the hollowcover; and provided at its upper, end with a cathode supporting cap 22having a central aperture through which the upper end of the The latteris suitably secured in the aperture of the cathode supporting cap 22 andis sealed therein against the admission of air by a sealing gland,indicated generally at 23, which also serves to insulate the cathodefrom the metal casing. Vaporous titanium, tetrachloride is adapted to befed from a supply sourcetnot shown) into the hollow solubilizationcathode by means of a feed tube 24 secured to the upper end of thecathode. A conductor 25 is alsoattached to the upper end of the hollowcathode for connecting the latter to a direct current source.

The anode 26 of the cell is preferably formed of graphite and issupported from the hollow cover of the cell by a sleeve-and-capassembly, indicated generally at 27 and 28 respectively, designed toinsulate the anode from the casing 10. Connected to the upper end of thegraphite anode 26 is a conductor 29 of a direct current source. Althoughnot necessary to the successful operation of the cell, the anode may bemade hollow, as shown, and in this case is provided with a closure capwhich is removable for providing access to the anode chamber of the cellwhereby additional salts may be added thereto without removal of thecell cover 11.

In order to prevent the chlorine formed at the anode from migrating toother parts of the cell, and, in particular, into contact with thecathodes, the upper end of the graphite anode is surrounded by aconcentric substantially impervious tubular gas-barrier 30, which may beformed of silica or an equivalent material and which extends from withinthe anode supporting sleeve 27 downwardly into the electrolyte of thecell to a point just below the surface thereof. Thus the chlorine whichis evolved at the anode will be confined within the sleevelike gasbarrier and conducted thereby upwardly to an exhaust port 31 adjacentthe cap 28.

In the embodiment of the invention shown the anode issupported by itssleeve-and-cap assembly substantially diametrically opposite thesolubilization cathode 20 but it will be understood that the position ofthe anode relative to the cathode is not critical. It is preferable,however, to arrange the two cathodes and the anode in a generallytriangular relationship such as shown in Fig. 2 and hereinafterdescribed.

The deposition cathode of the cell may comprise a solid rod 32 formed ofnickel or other suitable metal and is adapted to be supported in theelectrolytic bath of the cell by supporting means assembled in theaforesaid jacketed stack of the cell. In this respect the jacketed stack12 of the cover, the aforesaid solubilization cathode 20 and the anode26 are arranged substantially in triangular relationship, i. e. thesolubilization cathode 20, the anode 26, and the deposition cathode 32(the position of which corresponds to that of the jacketed stack 12) arearranged preferably substantially at the respective apices of anequilateral triangle.

The cathode rod supporting-means is preferably a rod 33 formed of copperor other suitable metal and detachably fastened at its lower end to theupper end of the deposition cathode 32. The upper end of the rod 33extends upwardly through the jacketed stack 12, from which it isinsulated and in which it is guided by centering-means (not shown), andis provided at its upper extremity with an eye or equivalent holdingmeans, indicated generally at 34, by which the deposition cathode 32 maybe selectively raised and lowered in the cell. Attached to the upperextremity of the cathode supporting rod is a conductor 35. In its lowerposition in the cell the deposition cathode 32 is adapted to be immersedin the electrolyte 36 of the cell at a point corresponding substantiallyto an apex of the equilateral triangle formed by the solubilizationcathode 20, the anode 26, and the deposition cathode.

As shown in Fig. 1, the jacketed stack 12 of the cell is providedadjacent its lower end with an air-lock, identified by the valve 37,whereby the jacketed stack may be closed off from the interior of thecell. Thus after a deposit of metal has accumulated on the cathode 32the latter may be drawn upwardly out of the electrolytic bath 36 intothe upper part of the jacketed stack 12, i. e. above the air-lock valve37. The latter is then closed whereupon the metal deposit on the cathodeis allowed to cool in an inert atmosphere which is provided in the upperend of the stack by feed pipes 38 and 39 adapted to circulate an inertgas, such as argon, therethrough. To accelerate the cooling of the metalon the cathode the stack is provided with a concentric jacket 40 havinginlet and outlet ports by which a coolant may be circulated through thejacket.

Since nitrogen, oxygen or an oxygen-containing constituent adverselyaifec the quality of the salt bath and the titanium metal at elevatedtemperatures it is preferred to carry out the reaction in an atmospheresubstantially free of oxygen, water vapour, nitrogen and the like, andto this end the hollow cover 11 of the pot is provided with inlet andoutlet pipes 41 and 42 respectively by which an atmosphere of argon oran equivalent inert gas may be maintained above the surface of the electrolyte in the cell. In the embodiment of the invention shown herein theaforesaid inlet and outlet pipes 41 and 42 respectively are locatedbetween the jacketed stack of the cover and the anode and cathoderespectively but it will be appreciated that any other arrangement maybe used.

Pursuant to the objects of the invention the improved cell is providedwith superior diaphragm means for obtaining high concentrations oftrivalent and divalent titanium values in the vicinity of thesolubilization cathode, and relatively low concentrations of thesereduced titanium values in the vicinity of the deposition cathode. Ingeneral, the total concentration of titanium trichloride and titaniumdichloride at the solubilization cathode is preferably from 6 to 8 molaland of the order or" from .2 to .4 molal at the deposition cathode, i.e. the concentration of low valent titanium values at the solubilizationcathode is from twenty to thirty times that in the remaining portion ofthe bath. Referring especially to Fig. 2 the diaphragms of the cell areindicated at 43 and 44 respectively, each diaphragm being substantiallyof the same dimensions and constructed of suitable porous electricallynon-conductive material which will withstand the temperature andcorrosiveness of the salt bath and will also permit the flow of ionstherethrough but retard the mixing, by convection, of ditferent portionsof the melt. Satisfactory results have been obtained by the use of fusedporous alumina diaphragms.

In the embodiment shown in Figs. 1 and 2 each diaphragm is asubstantially cylindrical cup-shaped member the diameter of which is ofthe order of two-tenths the diameter of the cell, the height of eachdiaphragm being such as to extend above the level of the electrolyte inthe cell. It is evident, of course, that the cylindrical diaphragms ofthe solubilization cathode and anode respectively serve to segregatecorresponding regions of the electrolytic bath and that the volume ofelectrolyte within, each of these segregated regions of the bath ismeasured approximately by the transverse area of the segregated regiontimes the height of the electrolyte. Thus in the embodiment shown thevolume of each segregated region of the bath is substantially onetwenty-third the volume of the non-segregated region of the bath. Forsmall installations the cylindrical cup-shaped diaphragms are quitesatisfactory but it will be understood that so long as the volume of theportion of the electrolyte segregated by each diaphragm, relative to thevolume of the remaining portion of the electrolyte is maintained withinthe limits specified, the arrangement and configuration of thediaphragms may very depending upon cell size, shape and related factors.

Both d'iaphragms are supported on the bottom of the cell, the diaphragm43 being arranged substantially concentric with the solubilizationcathode and the diaphragm a 44 being arranged substantially concentricwith the anode. The diaphragm 43 of the solubilization cathode thusprovides a relatively small segregated region in the electrolytic bathin which from 6 to 8 molal of reduced titanium values are initiallyconcentrated. This region is hereinafter identified as thesolubilization region a of the cell. During the operation of the cellthe level of the electrolyte in the region a will be raised andmaintained higher than the level of the electrolyte in that portion ofthe cell outside of the cathode diaphragm and hence the concentration ofreduced titanium values within the segregated solubilization region a ofthe cell will be urged, by the difference in hydrostatic pressure, topass through the porous diaphragm 43 into the main portion ornon-segregated region of the bath in which the deposition cathode isimmersed. This region of the bath is hereinafter identified as the metaldeposition region b and, as stated above, the volume of thisnon-segregated region of the electrolytic bath is substantailly twentythree times the volume of the segregated solubilization region 0. Thusas the reduced titanium values pass through the diaphragm 43 into themetal deposition region b of the electrolytic bath they are widelydispersed and hence the concentration of these reduced titanium valuesin the bath surrounding the anode diaphragm and the deposition cathodeis relatively low, i. e. of the order of 0.2 to 0.4 mo-lal therebyenhancing the deposition of coarse crystalline titanium metal on thelower end of the deposition cathode and precluding titaniumtetrachloride losses at the anode.

While the relative diameters of diaphragms and cell specified above havebeen successfully employed in the electrolytic production of titaniummetal it will be understood that the specific relationships given areillustrative only and that other size relationships may be used toobtain a high concentration difierential of reduced titanium values inthe bath within the range specified herein.

The diaphragm 44 surrounding the anode not only serves to prevent thereduced titanium values in the metal deposition region b of the bathfrom contacting the anode but also, in conjunction with the gas barrier30, serves to prevent the chlorine gas, which evolves at the anode, fromescaping to either the solubilization or deposition cathode. To this endthe upper end of the anode diaphragm fits closely about the lower end ofthe abovedescribed gas barrier 30 whereby the chlorine gas is carried edby the latter from the cell to a suitable collecting device.

In addition to the above described functions of the diaphragms of thecell, bipolar electrode effects are substantially eliminated bypreventing any sludge, i. e. finely divided titanium metal, which may beformed in the vicinity of the solubilization cathode from migratingtowards the anode. In fact it has been observed that any sludge whichmay form at the solubilization cathode is confined to the solubilizationarea a by the diaphragm 43 and is dissolved or partially dissolved uponthe subsequent or continuous addition of titanium tetrachloride vapor.

The electrolyte 36 partially fills the cell and comprises a fused saltconsisting essentially of a molten halide mixture of an alkali oralkaline earth metal including magnesium, and, in particular, thechlorides which may be employed singly or in combination. Mixtures ofthese halides which form low melting point eutectics are preferred as,for example, mixtures of sodium chloride and strontium chloride orsodium chloride and magnesium chloride. For optimum operation of thecell, the electrolyte should be maintained at a temperature from about670 C. to 750 C. and in the embodiment shown the heating means of theelectrolyte comprises a furnace indicated generally at 45, in which thecell is mounted. It will be understood, however, that the electrolytemay be heated by other means such as, for example, electricallyenergized heating electrodes mounted in the cell.

In operating the cell to produce titanium metal, it is preferred todistribute the total of substantially four faradays of current, per moleof titanium tetrachloride intro.- duced into the cell,substantiallyevenly between the two cathodes, that is to say,substantially two 'far-adays .to the solubilization cathode andtwofaradays to the deposition cathode. Moreover, in order to minimize theformation of sludge at the solubilization cathode due to reduction of aportion of the titanium tetrachloride directly to metallic titanium, itmay be desirable to add slightly less "than two faradays at thesolubilization cathode per mole of titanium tetrachloride so as toinsure the presence of some trivalent titanium values in the segregatedregion a surrounding the solubilization cathode. Assuming that theelectrolytic bath has "been heated to from 670 C. to 750 C. and thevaporous titanium tetrachloride is being introduced into the baththrough the hollow cathode, the reduction of titanium tetrachloridethrough the trivalent and divalent titanium values to titanium metal isaccomplished rapidly and efficiently, due to the above describedarrangement of di-aphragms. Following the accumulation of a relativelylarge deposit of metal on the deposition cathode, the deposition cathodeis withdrawn from the electrolytic bath up into the upper part of thecooling stack whereupon the air-lock 37 is closed and the metal depositis cooled in an atmosphere of an inert gas such as, for example, argon.After the deposit has cooled sufficiently to avoid .embri-ttlement uponexposure to the atmosphere, the sealed cover of the stack is removed andthe deposition cathode is withdrawn from the upper end thereof. A newcathode is then introduced into the upper end of the stack above the airlock, whereupon the upper portion of the stack is closed and purged ofair by circulating argon or a similar inert gas therethrough. Thereafterthe air-lock is opened and the new cathode is lowered into theelectrolyte for the deposition of additional metal thereon. 1

It has been observed that because of the relatively large depositionarea b of the electrolyte little or no crust is formed on the surface ofthe bath and hence inadvertent stripping of metal from the depositioncathode is avoided. Since there will be some removal of the electrolyticbath each time a deposit of metal is withdrawn, the bath should bereplenished periodically which may be done by removing the cover orpreferably by introducing additional salts into the cell by way of thehollow anode.

While a cell of the design shown in Figs. 1 and 2 is convenientespecially for relatively small installations and has producedremarkably large quantities of the highest quality titanium metal, itwill be understood that modifications of the cell and its diaphragms areincluded within the purview of the invention. Thus as shown in Figs. 3and 4 the invention may be embodied in a cell comprising an outer sealedcasing 50 of welded steel con struction and lined with refractory brick51, or an equivalent material to form a cell-well 52 in which thecathodes, anode and diaphragms are mounted. As pointed out above foroptimum performance and efficiencies it is preferred to arrange thecathodes and anodes in substantially triangular relationship and hencein this embodiment of the invention the transverse section of the cellis a substantially equilateral triangle. The hollow solubilizationcathode 53 is supported by the cover adjacent an apex of the cell-well,the anode 54 is supported by the cover adjacent a second apex of thecell-well while the deposition cathode 55 is located roughly in theregion of the third apex of the cell-well in the manner shown in Fig. 4.The structural details of the supporting means for the respectivecathodes and the anode may be substantially identical to thosehereinabove described and hence a detailed description thereof will beunnecessary. Suifice to say that a conductor 56 and feed tube 57 areattached to the upper end of the hollow solubilization cathode which issupported in the cover by an insulated sleeve 23; the deposition cathode55 is supported by an insulated :rod 58, having a conductor 59 at itsupper end and mounted in a jacketed stack 60 which extends upwardly fromthe top of the casing and is of the construction shown in Figs. 1 and 2;while the anode is provided at its upper end with a conductor 61, and issupported by a sleeve-and-cap assembly, indicated generally at 62,adapted to carry off the chlorine gas developed at the anode.

The cathode and anode .diaphragrns of the cell are shown at 63 and 64respectively each being arranged to segregate the correspondingapex-portion oi the cell-well from the remaining portion thereof, thearrangement of the diaphragm 63 of the solubilization cathode being suchthat the areaa segregated thereby from the deposition portion b" of thecell-well is substantially one-seventh the volume of the latter wherebya relatively high concentration differential of from twenty to thirtytimes of reduced titanium values is maintained between thesol-ubilization cat-holyte and the deposition c-atholyte.

More particularly the diaphragrns 63 and 6d are substantiallyrectangular plates of refractory electrically non-conductive materialsupported by their vertical edges in indentations or grooves incorresponding side walls of the cell-well. The lower ends of thediaphragms are shown seated on the bottom of the cell-well with theupper ends of the diaphragms extending above the surface 'of the saltbath 65. As shown in Fig. 3 the upper end of .the anode diaphragm isadapted to engage in a groove in the bottom edge of a gas-barrier 66which comprises a substantially rectangular plate of an imperviousrefractory material.

The heating means for the cell is shown as comprising, in this instance,a pair of electrodes 67 and 68 which project through the walls of thecell adjacent the bottom .of the cell-well as shown especially well inFig. 4-, the electrode 67 being located in a side wall of the cellsubstantially midway of the cathode 53 and anode 54, while the electrode68 extends into the cell-well substantially opposite the electrode 67.

The operation of the modified cell is substantially the same as that ofthe cell hereinabove described.

The invention may be carried out in other specific ways than thoseherein set forth without departing from the spirit or essentialcharacteristics of the invention and the present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claim are intended to be embraced therein.

We claim:

An electrolytic cell for the production of a refractory metal bysolubilization and reduction of a refractory metal halide in a fusedsalt bath, said cell comprising: a casing adapted to contain said bath;a cover on said casing arranged to seal said casing from the atmosphere;an anode; a hollow solubilization cathode, a deposition cathode andmeans to feed said refractory metal halide through said hollow cathodeinto said fused salt bath; supporting means carried by said cover andarranged to suspend said anode and said cathodes in said bath insubstantially equally spaced triangular relationship in said casing; andelectrically non-conductive cup-shaped diaphragms supported in saidcasing and circumscribing said anode and said hollow solubilizationcathode respectively to form a sol'u'oilization region in said bathsegregated from said anode and from the non-segregated region of saiddeposition cathode, the volume of said solubilization region being aboutone twenty-third the volume of the non-segregated region of said bathwhereby the concen' tration of reduced metal halides in the segregatedregion of said hollow solubilization cathode is of the order of twentyto thirty times greater than the concentration of reduced halides in theportion of the bath surrounding said deposition cathode.

References Cited in the file of this patent UNITED STATES PATENTS1,371,698 Linder Mar. 15, 1921 FOREIGN PATENTS 682,919 Great BritainNov. 19, 1952 81,510 Argentina Sept. 25, 1951

